The present invention relates to a diffractive optical element, objective optical system, optical pick-up device and optical information recording reproducing apparatus, and particularly, to an optical pick-up device and optical information recording reproducing apparatus by which, by using light fluxes projected from light sources of different wavelengths, the information can be recorded and/or reproduced for optical information recording media, and to a diffractive optical element and objective optical system appropriate when used for them.
Conventionally, an optical pick-up device which can record/reproduce for a plurality of kinds of optical disks having different recording densities, is well-known. For example, there is a device by which a DVD (Digital Versatile Dik) and a CD (Compact Disk) are recorded/reproduced by using an optical pick-up device. Further, recently, as an optical disk having different recording densities, an optical pick-up device which is compatible with a high density optical disk (hereinafter, optical disks which use a blue violet laser light source as a laser light source for recording/reproducing, are generally called “high density optical disk”), using the blue violet laser light source (for example, a blue violet semiconductor laser or a blue violet SHG laser), and the conventional DVD, further, also with the CD, is required.
As the optical pick-up device having the compatibility with the high density optical disk and the DVD, as written in the following Patent Documents, a technology using a diffractive optical element on which a diffractive structure composed of a plurality of ring-shaped zones around the optical axis is formed, is well-known.
[Patent Document 1]Tokkkai 2001-60336[Patent Document 2]Tokkkai 2002-298422[Patent Document 3]Tokkkai 2001-93179
The technology written in the above Patent Documents is a technology which uses the lower diffraction order beam than the diffraction order of the beam for the recording/reproducing of the high density optical disk as the recording/reproducing beam, in a manner that a second order (or a third order) diffractive-light is used as the beam for the recording/reproducing of the high density optical disk, and a first order (or a second order) diffractive light is used as the beam for recording/reproducing of DVD. According to this technology, to the recording/reproducing beam of respective optical disk, while securing the high diffraction efficiency, the variation of the focus position to the minute wavelength change in the blue violet wavelength range can be suppressed.
In this manner, for the purpose to secure the high diffraction efficiency to both of optical disks having the large wavelength difference of the beam for the recording/reproducing, when a wavelength of the beam for the recording/reproducing of the high density optical disk is λ1, and a wavelength of the beam for the recording/reproducing of DVD is λ2, and the refractive indexes of the diffractive optical element to the wavelengths λ1, λ2 are respectively N1, N2, it is necessary that a combination of the diffraction order n1 of the wavelength of the beam for the recording/reproducing of the high density optical disk, and the diffraction order n2 of the wavelength of the beam for the recording/reproducing of DVD, is selected, so that a ratio δφD of the optical path difference added to the light flux of the wavelength λ1 by the diffractive structure defined by the following equationδφD={n1·λ1/(N1−1)}/{n2·λ2/(N2−1) }  (18)and the optical path difference added to the light flux of the wavelength λ2 is a value close to 1.
However, in the diffractive sructure which generates the diffraction light of the combination of the diffraction order n1 and the diffraction order n2, because a changing amount of the spherical aberration per unit wavelength becomes large, the selection of the laser light source becomes necessary, and there are a problem which results in, an increase of the production cost of the laser light source, and an increase of the production cost of the optical pick-up device, and a problem that, because the spherical aberration of the objective optical system largely changes by the wavelength change of the laser light source following the temperature change, the stable recording/reproducing characteristic for the high density optical disk can not be obtained.
Because the changing amount of the spherical aberration per such a unit wavelength change is increased in proportion to 4th powers of a numerical aperture (NA) of the objective optical system, particularly, in the objective optical system of the numerical aperture (NA) 0.85 and the high density optical disk of a standard using a protective layer of 0.1 mm (hereinafter, called 0.1 mm standard), as represented by a blue ray disk which is a standard of the high density optical disk, the above problems are more actualized.